1. Field of the Invention
This invention relates to the field of external defibrillators. In particular, the present invention relates to a high voltage phase selector switch for providing biphasic defibrillation pulses to a patient.
2. Description of the Related Art
Cardiac arrest, exposure to high voltage power lines and other trauma to the body can result in heart fibrillation which is the rapid and uncoordinated contraction of the cardiac muscle. The use of external defibrillators to restore the heartbeat to its normal pace through the application of an electrical shock is a well recognized and important tool for resuscitating patients. External defibrillation is typically used in emergency settings in which the patient is either unconscious or otherwise unable to communicate. Time is of the essence since studies have shown that the chances for successful resuscitation diminish approximately ten percent per minute.
Commercially available defibrillators such as those available from SurvivaLink Corporation, the assignee of the present application, are currently configured to produce monophasic waveform defibrillation pulses. Monophasic (i.e., single polarity) pulses such as a damped sine waveform and a truncated exponential waveform have been demonstrated to be effective for defibrillation, and meet standards promulgated by the Association for Advancement of Medical Instrumentation (AAMI). Electrical circuits for producing monophasic waveform defibrillation pulses are generally known and disclosed, for example, in the Persson U.S. Pat. No. 5,405,316 which is assigned to the assignee of the present invention and the disclosure of which is herein incorporated by reference.
The efficacy of biphasic waveform pulses (effectively two successive pulses of opposite polarities) has been established for implantable defibrillators. For example, studies conducted on implantable defibrillators have shown that biphasic waveform defibrillation pulses result in a lower defibrillation threshold than monophasic pulses. A variety of theories have been proposed to explain the defibrillation characteristics of biphasic waveform pulses but no definite conclusions have been reached.
It is anticipated that the efficacy and advantages of biphasic waveform pulses that have been demonstrated in implantable defibrillators will be demonstrated in external defibrillators as well. It has been known to use electromechanical vacuum or gas filled relays to switch the output of storage devices to form biphasic waveforms. These devices are electrically suitable for use in external defibrillators, but pose practical problems in that they are generally fragile, large and very expensive. One important shortcoming is that such devices are not suitable for breaking or interrupting large voltages and currents and, when called upon to do so, often damage the relay contacts. External defibrillators output defibrillation pulses in the range of 2000-3000 volts. The typical load for external defibrillators are in the range of approximately 25-225 ohms. At these voltages and resistances, the circuits must be able to handle currents in excess of 100 amps. This shortcoming is particularly significant when it is desired to truncate a first portion of a biphasic defibrillation pulse. In such circumstances, to truncate the pulse or waveform without damage to the contacts, it is known to short-circuit the capacitor bank supplying the pulse to be terminated or truncated. Such an approach suffers from the further shortcoming that the energy short-circuited is lost to the system, increasing inefficiency and adding electrical (and mechanical) stress to the components carrying short-circuit current. Thus, there is a continued need for a low cost, compact, and rugged switching circuit.